It is known to use the ratio of two diameters between an input and an output to produce a CVT-type transmission. The motion transmission between the input and the output occurs by friction.
Thus, DE-A-10 2006 016 955 and FR-A-2,173,528 disclose variable speed drives in which two bells cooperate with a planet gear that bears against the inner surfaces of those bells and whose angular position around an axis perpendicular to and not intersecting the axis of rotation of the bells makes it possible to adjust the transmission ratio of that variable speed transmission. The position of the planet gear relative to the inner surfaces of the bells is adjusted by sliding the planet gear relative to those surfaces, perpendicular to its direction of rotation. During that sliding, the means for controlling the position of the planet gear must overcome a frictional force between the planet gear and the inner surfaces of the bells. To avoid excessive stress on those control means, that frictional force should therefore be relatively low. Furthermore, for effective transmission of the movement within the variable speed transmission, it is important to limit the slipping between the input and output elements, i.e., to work with a relatively high friction coefficient between the planet gear and those surfaces.
There are therefore two opposite constraints relative to the friction coefficient between the planet gear and the inner surfaces of the bells, which requires compromises and is detrimental either to the lifetime of the variable speed transmission or to its efficiency. Furthermore, in these known variable speed transmissions, adjusting the angular position of the planet gear is relatively time-consuming, since it is necessary to account for the slipping to be done between the planet gear and the inner surface of the bells, that slipping not being able to be immediate in light of the friction between those parts. Furthermore, this slipping of the planet gear, perpendicular to its direction of rotation when its position is being adjusted, tends to wear the planet gear and/or the inner surface of the bells out.